The present invention relates to a sound muffling material. The material is intended particularly although not exclusively for use in mufflers and silencers fitted to internal combustion engine exhausts.
Exhaust mufflers generally include a sound muffling material, usually glass fibers. This material acts to attenuate sounds transmitted through the exhaust system. The fibers are usually disposed in at least a part of the muffler. The fibers generally fill a part of the muffler to a certain density to achieve an effective muffling effect. The fibers are usually in a volumized form.
In one existing arrangement an exhaust muffler includes a cylindrical steel body, usually referred to as a box, in which there is disposed coaxially a perforated steel tube. The perforated steel tube is mounted on annular end caps which are affixed to opposite ends respectively of the cylindrical body by welding or crimping. A muffling material, usually glass fibers, is disposed in the annular region between the perforated steel tube and the muffler body. In use exhaust gases are directed through one end cap, along the perforated tube, and out the opposite end cap.
Mufflers of this type are assembled in one of two common ways. Generally the muffler is assembled by attaching one end cap to support the perforated tube. In one method volumized continuous filament glass fibers are injected, through the open end of the muffler body, into the annular region between the perforated tube and muffler body using specialist equipment. In another method a glass fiber needlefelt fabric is provided wrapped around a cardboard tube, or former, of a similar diameter to the perforated metal tube in the muffler. The cardboard tube is positioned above the perforated tube and the cylinder of needlefelt fabric slid off the cardboard tube and onto the perforated tube.
There are a number of problems associated with both the above described methods. The equipment used to inject continuous filament fibers is expensive and therefore limits the number of mufflers that can be produced simultaneously at reasonable cost. When fibers are provided in the form of a needlefelt they are not necessarily continuous filament fibers. As such, when the muffler is used some fibers may pass through the perforated tube and into the flow of exhaust gases. This is undesirable as the effect of the muffler will be diminished and the escaping fibers may cause problems in the remainder of the exhaust system. Also, sliding a cylinder of needlefelt fabric onto a perforated tube is also inconvenient as the fabric tends to snag on the cut end of the perforated tube. Further, the cardboard tubes present a waste management problem. Often, the tubes are re-used which necessitates returning the tubes to the supplier, increasing transport costs.
By far the most significant drawback with conventional methods of filling mufflers with fibers is that where the fibers are loose, especially in the case of the preferred continuous filament fibers, and the muffler is filled with the required density of fibers this presents problems when the end cap is attached to the muffler. Where fibers stray out of the muffler body they may become trapped between the muffler body and the end cap. This adversely affects the quality of the join between the end cap and muffler body both when the end cap is attached by welding and crimping. It is therefore essential that before the end cap is attached to a muffler body the fibers disposed in the body are carefully moved from the region of the join. This is tedious and time consuming.
Another type of muffler is the clam shell type, which includes two portions which are crimped or welded together to form a complete unit. Mufflers of this type are produced in a variety of shapes and sizes, in general, however, each half is relatively shallow. The clam shell type of muffler cannot be easily filled with fibers using the above described methods as the fibers easily escape. Instead, short fibers are provided packed in, or continuous filaments injected into, perforated polythene bags. A bag of fibers is placed into one half of a clam shell muffler and the second half is welded or crimped to the first half. In use, high temperature exhaust gases cause the polythene bags to disintegrate, releasing the fibers. Again, there are problems associated with this technique. Firstly, the bags tend to be bulky in order to provide the correct density of fibers to fill the muffler. This makes joining the two halves of the muffler difficult. Secondly, where the bag is filled with short filament fibers problems are experienced with the fibers escaping from the muffler in use, as described above.
EP 0434895A discloses a silencer for an internal combustion engine comprising a hollow housing containing a web of fibers and a pipe extending therethrough. The web of fibers is confined by a plastics film and there is a substantial clearance space between the film and the housing. When the silencer has been connected with an internal combustion engine and is subjected to flow of hot exhaust gases from the engine the plastics film inside the silencer is destroyed so that it no longer confines the web of fibers.
DE 3827863A discloses an exhaust gas purification device which includes a resilient support mat. The resilient support mat is surrounded by a covering sheet such that it is compressed. In one arrangement overlapping marginal areas of the covering sheet are fixed together with blobs of adhesive which melts on heating to permit separation of the edges and expansion of the support mat.
WO 91/19082 discloses a protective material for a catalytic convertor block comprising a pad of fibrous material in an envelope of non-woven textile material. The envelope has its depth reduced in at least localized areas or positions by drawing together of opposing faces by stitching.
It is an object of the present invention to provide a convenient method of filling a muffler with fibers, particularly to enable continuous filament fibers to be easily used in clam shell type mufflers.
According to a first aspect of the present invention there is provided a sound muffling material comprising volumised continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200 kg/m3 by a material of lower softening temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated.
According to a second aspect of the present invention there is provided a sound muffling material comprising volumised continuous filament fibers retained in a compressed state in the form of a knitted or woven fabric with a density of at least 200 kg/m3 by a material which breaks down at a lower temperature than the fibers, arranged so that the material of lower softening temperature will release the fibers when heated.
According to a third aspect of the present invention there is provided a method of making a sound muffling material comprising the steps of providing continuous filament fibers, volumising the fibers, providing a material with a lower softening temperature than the fibers, compressing the volumised fibers and retaining the volumized in a compressed state by means of the material of lower softening temperature by forming the volumized fibers into a knitted or woven fabric with a density of at least 200 kg/m3.
According to a fourth aspect of the present invention there is provided a method of making a sound muffling material including the steps of providing continuous filament fibers, volumising the fibers, providing a material with a lower breakdown temperature than the fibers, compressing the volumized fibers and retaining the volumized in a compressed state by means of the material of lower breakdown temperature by forming the volumised fibers into a knitted or woven fabric with a density of at least 200 kg/m3.
According to a fifth aspect of the present invention there is provided a method of filling an exhaust muffler with fibers including the steps of placing a material according to either of the first or second aspects of the present invention into an exhaust muffler and heating the material so as to release the fibers.
According to a sixth aspect of the present invention there is provided a method of mounting an exhaust catalyst brick comprising the steps of wrapping the brick in a material according to either of the first or second aspects of the present invention and heating the material so as to release the fibers.
The material is preferably adapted for insertion into an internal combustion engine exhaust muffler, including both domestic and commercial vehicles as well as industrial applications, for instance silencers used on gas turbine installations and during jet engine testing. The material may also be used for catalyst brick support in exhaust systems.
The fibers are preferably heat resistant and may include silica, glass, mineral or basalt man made fibers. The fibers preferably comprise e-glass (electrical glass) fibers. The fibers are also preferably resistant to exhaust gases.
The fibers are preferably resistant to thermal breakdown at temperatures up to 500xc2x0 C., more preferably 1000xc2x0 C., still more preferably 1100xc2x0 C. or higher.
The average length of the fibers is preferably greater than 400 mm.
The fibers may be volumised by the process known as air texturising or volumizing.
The fibers may be volumized by using conventional compressed air operated volumizing equipment to separate the filaments in multi-filament strands or yarns, for example multiple fibre roving. The volume occupied by the fibers is preferably increased by at least a factor of ten. The fibers may also be texturized, again using conventional equipment, for example air-jet texturizing equipment.
The volumized heat resistant fibers are preferably retained, when in compressed form, so as to minimize their volume.
The volumized heat resistant fibers are preferably retained by an organic or synthetic material with a softening/melting point of lower temperature than that of exhaust gases, more preferably less than 200xc2x0 C., still more preferably below 150xc2x0 C. The retaining material preferably includes a fiber, for example a nylon polypropylene, polyethylene or polyester fiber. It is to be understood, however, that natural materials and fibers which breakdown at temperatures below the softening or breakdown temperature of the heat resistant fibers could be used, for example cotton fibers.
More generally the heat resistant fibers and retaining material are preferably chosen so that in use, for example in an exhaust muffler, the high temperature exhaust gases cause the retaining material to breakdown to release the heat resistant fibers. This allows mufflers and other equipment to be easily assembled with heat resistant fibers in a compressed form. As such the fibers take up a minimum of volume this overcomes the problem of stray fibers interfering with the assembly of the muffler and the difficulty associated with the insertion of bulky fibers into a muffler. When the muffler is first used the fibers are released and expand to fill the muffler in a desired manner.
In a preferred arrangement the heat resistant fibers are formed into a crochet or rochel knit fabric, retained by a lower melting point thread, for example a xe2x80x98sacrificialxe2x80x99 catch thread. The fabric may however take other forms, for example a woven fabric where the warp and weft include respectively heat resistant and heat softening fibers, or vice versa. Braided, twisted or netted methods of manufacture may also be used.
Fabrics according to the present invention may be configured so that upon the melting/breakdown of the retaining material the fabric expands in a predetermined manner. For example a strip of fabric may be arranged so that it will expand mainly in length and thickness but less so in width. This is a useful feature where the fabric is used in a cylindrically bodied muffler.
When the arrangement of fibers comprises a fabric it is preferable that that fabric has a density of at least 400 kg/m3, in compressed form, before softening/breakdown of the retaining threads.
When the arrangement of fibers is a fabric this may be produced continuously and cut into pieces of desired length. It is preferable that the ends of the fabric are secured to prevent fraying and premature expansion, for example by taping the ends of the fabric or using a thread lock adhesive. It is preferred that any tape or adhesive has a softening/thermal breakdown temperature of a similar order to the retaining material and in any event lower than that of the heat resistant fibers.
Portions of material of the present invention may be packed in plastic bags to aid handling. Such bags preferably breakdown on exposure to heat in exhaust systems.
The present invention provides an improved method of and muffling material for filling exhaust mufflers. The method dispenses with the need for the use of either expensive equipment or for cardboard formers or other packaging. As the fibers are provided in compressed form they take up less volume and are therefore considerably easier to insert into muffler boxes. As the fibers are retained they are also less likely to interfere with the closing of muffler boxes by crimping or welding.
Forming the fibers into a fabric provides the ability to control accurately the density of infill of muffler boxes and the like. They also allow a much higher overall fill density of fibers to be achieved than with conventional materials and methods.
Where continuous filament fibers are provided this reduces the tendency of fibers to escape into an exhaust system.
Fabrics may also be used as a catalyst support mat for catalyst brick support. Catalyst bricks cannot be welded. Fabrics can be used to retain catalysts in exhaust systems by wrapping the catalyst brick in a fabric, the wrapped catalyst brick is then placed in a part of an exhaust system, often similar to a muffler box.
Where fabrics according to the present invention are employed they can be arranged to expand on initial heating to firmly secure a catalyst brick in place and take account of the differential expansion of the catalyst brick and housing. This minimises any movement of the catalyst brick, caused, for example, by vibration of an exhaust system, and so prolongs catalyst life.